A variety of systems have been proposed and used for adding communication data capability to radio navigation signals as described, for example, in U.S. Pat. Nos. 4,800,341 and 4,821,038 of common assignee herewith, and publications discussed therein.
A significant improvement in expanding the data communication speed or digital bit rate (at least from 70 bps to over 250 bps) for communication added to Loran-C radio navigation pulse trains also without affecting the navigation capability and integrity thereof; is described in my earlier copending patent application for Method Of And Apparatus For Expanding The Digital Bit Rate Potential For Communication Added To Loran-C Radio Navigation Pulse Trains And The Like, Without Affecting The Navigation Capability And Integrity Thereof, Ser. No. 09/833,022, filed Apr. 11, 2000, now U.S. Pat. No. 6,452,547, issued Sep. 17, 2002.
In this copending application, the Loran-C signal is frequency-modulated by tuning or sweeping the high-Q Loran-C antenna frequency between predetermined desired frequencies by varying series inductance and series capacitance at the antenna in steps, by means of fast, high-power, solid-state switches. This demonstrated feasibility initiated serious discussion as to the real potential of such intrapulse frequency modulation (IFM) for expanding the data communication capability of Loran-C navigation pulses.
A preferred and novel solid-state switching methodology and apparatus ideally suited for such Loran-C IFM was then disclosed in my further copending patent application, Method of High-Power Switching And Switch Apparatus For Frequency Modulation Within Loran-C Signals, Ser. No. 09/922,283, filed Aug. 3, 2001. This system embraced improved frequency—modulation switching apparatus for rapidly increasing and decreasing the frequency within the radio-frequency pulses of the Loran-C radio wave pulse trains transmitted by an antenna having series inductance and capacitance switchable into and out of circuit therewith. The apparatus comprises a solid state four-terminal rectifier bridge circuit with opposing pairs of bridge terminals connected with one pair of opposing terminals shunting said inductance and said capacitance; and series-connected staturable and linear inductors and an SCR switch connected between the other pair of opposing terminals of the bridge circuit, whereby the high-speed triggering of the SCR switch on, effects corresponding high-speed frequency increasing or decreasing of the frequency within the radio-frequency pulse to provide the desired frequency modulation therein. The switch turns off at the end of the radio-frequency pulse tail when the SCR switch current drops below its holding current.
Following these advances that have practically enabled increased data communication speed with such IFM applied to Loran-C navigation pulses, it became evident that effective back-up for the satellite WAAS system was now available with Loran-C pulses capable of transmitting the full WAAS message of 250 bits/500 symbols in one second, as described by Ben Peterson et al in “High Speed Loran-C Data Communications-2001 Update” appearing in Proceedings of Second International Symposium Integration of LORAN-C/Eurofix and Egnos/Galileo-Loran 2001, presented at Bonn, Germany, February, 2001. In later-described FIGS. 1 and 2, proposed IFM schemes of Peterson et al are shown involving specific phase “arys”, wherein a plan or scheme of frequency hopping within the Loran-C pulse is effected, with the phase shifts at different times during the pulse measured, uniquely identifying the particular IFM ary, as later more fully explained. (The term “n-ary” function is defined as a function with any number n of arguments, by analogy to unary, binary, etc.).
In accordance with the present invention, however, a vastly improved IFM ary than such prior proposals is provided that enables less out-of-band energy to be obtained and with smaller frequency shifts required, that thus reduces the required voltage rating of the switches. The present invention also provides a novel circuit apparatus for implementing IFM schemes.